A variable-orifice pneumotachograph is described. It has a resistance that is relatively constant over a flow range from 0.1 to 2.5 liter/sec so it puts out a differential pressure signal that is directly proportional to flow. It is specifically designed for monitoring respirator patients and sacrifices some of the accuracy of a laminar flow (Fleisch) type device to gain greatly improved immunity to artefacts caused by airway water.